The invention to which this application relates is an improved valve construction. In particular the invention is related to improvements for the type of valve known in the trade as a double block or a double block with bleed valve (hereinafter both referred to as a double block valve). This type of valve is used in pipelines as an isolation valve and comprises first and second ball valve assemblies in an in line configuration. The provision of the two ball valve assemblies means that if one of the assemblies fails there is still another valve assembly operational.
The double block valve is commonly connected to pipelines in process, oil and gas and instrumentation industries such as refineries for carrying compressible, incompressible, or combinations of these fluids. The use of the valve is well known in the trade and can be provided to allow safe isolation of parts of the pipeline to allow repair or maintenance work and/or the connection of monitoring and/or injection apparatus. The valve is normally provided with a vent facility which is located between the two valve assemblies so as to allow for the controlled venting of the fluid carried in the pipeline and the collection of samples.
There are many different designs of double block valve assemblies but generally they comprise a valve body in which the valve components are held, with a vent formed therein. The valve body is provided with a passage therethrough, and into said passage, and from each end thereof, are fitted the respective valve assembly components of the ball, seats and seals. When the components are fitted into the passage from each end, the components are locked in position by the insertion of retaining members into each end of the passage. This design of valve has been used for a considerable period of time but has several disadvantages, in that the valve is relatively long and bulky and also that the valve has at least two joints where leakage or failure of the valve can occur. In the example described the joints are created between the retaining means inserted at each end of the passage, at which failure and leakage can occur. It is accepted that the greater the number of joints in the valve then the greater the risk of leakage occurring and, when one considers that the fluids being carried can be hot, toxic, corrosive and/or combustible, it will be appreciated that any leakage can be catastrophic.
Patent application no. 2271164 discloses an arrangement for a double block valve where there is provided a sealing element which is inserted into the valve body and which acts as a common element for both of the ball valve assemblies. This is claimed to reduce the overall length of the double block valve assembly. However the valve according to this patent still has at least two joints at which leakage can occur, these being between each of the assemblies and the intervening common valve seat.
GB2064730 describes a butterfly valve arrangement having a pair of rotateable flaps internally of the valve which are operated by a single actuation means and close simultaneously. A venting arrangement is provided in conjunction with the flaps such that the venting arrangement is opened to allow for drainage when the two flaps and thus the valve is closed. The valve is stated as being for the food and drinks industries, and is insubstantial in that a significant fluid pressure build up behind either of the flaps may burst same causing catastrophic failure.
DE3339472 discloses a valve consisting of two independently operable ball valves encased within a single housing and further having an adjustable resilient elastomeric member between and contacting the ball valve assemblies to create a seal therebetween. The resilient member is elongated between the ball valves to compress same by means of driving one or more arrow-headed members transversally into the member to deform same longitudinally. The valve is not formed from two separable assemblies, and cannot therefore be used in the gas and oil industries.
U.S. Pat. No. 4,335,747 describes a valve having two rotateable ball valves housed in separate assemblies, one of said ball valves being substantially spherical and the other being generally spherical but having a spherically arcuate recess which receives a portion of the other substantially spherical ball valve. In the closed condition, the partially spherically shaped ball valve is prevented from being rotated by the spherically shaped ball valve, and therefore not only is the operation of the ball valves dependent on one another, the removal of one of the assemblies will affect the integrity of the remaining assembly because in the closed condition, one ball valve provides support for the other.